1. Field of the Invention
The present invention relates to an array of field emission cathodes.
2. Description of the Prior Art
There is an array of minute field emission cathodes, each element having a cathode of several microns in size. It is known as the Spindt-type field emission cathode, which will be explained with reference to FIG. 11.
Referring to FIG. 11, there is shown an electrically conductive substrate 1 made of silicon or the like, which serves as a first electrode. On the substrate 1 is a sharply pointed conical cathode 9 made of such a metal as tungsten and molybdenum, which has a high melting point and a low work function. Around the conical cathode 9 is an insulating layer 2 made of SiO.sub.2 or the like. On the insulating layer 2 is a second electrode 3 (as a gate electrode or a counter electrode of the cathode 9) made of a high-melting metal such as molybdenum, tungsten, and chromium. There is an alternative structure in which a first electrode 11 is formed separately on a substrate 10 as shown in FIG. 12.
An array of field emission cathodes mentioned above is produced by the process explained below with reference to FIG. 13. As shown in FIG. 13A, the process starts with forming consecutively on a silicon substrate 1 an insulating layer 2 of SiO.sub.2 (1-1.5 .mu.m thick) by CVD (chemical vapor deposition), a metal layer 3a of a high-melting metal such as molybdenum and tungsten (in thickness of the order of thousands of angstroms, say 4000 .ANG.) by vacuum deposition or sputtering, and a resist 4 by coating.
As shown in FIG. 13B, the resist 4 is subsequently exposed and developed by photolithography to form an opening 5a, about 1 .mu.m in diameter (indicated by w). The metal layer 3a undergoes anisotropic etching through the opening 5a by RIE (reactive ion etching) to form an opening 5 of the same diameter as the opening 5a. Thus there is formed a gate electrode 23 from the metal layer 3a. The insulating layer 2 undergoes over-etching through the opening 5 to form a cavity 6. This over-etching is carried out such that the periphery of the opening 5 of the gate electrode 23 projects from the inside wall of the cavity 6 in the insulating layer 2.
As shown in FIG. 13C, an intermediate layer 7 is formed on the gate electrode 23 by oblique deposition in the direction of arrow a (at such an angle as to avoid deposition in the opening 5 and cavity 6), with the substrate 1 turning. This intermediate layer 7 is made of aluminum or nickel, which can be removed later by etching. The angle of oblique etching should be 5.degree.-20.degree. with respect to the surface of the substrate 1. The oblique deposition takes place such that the intermediate layer 7 has an opening which is smaller than the opening 5.
As shown in FIG. 13D, a material layer 8 of molybdenum or the like is deposited over the entire surface by vertical deposition so as to form a conical cathode 9 in the cavity 6. (Since the opening in the intermediate layer 7 is smaller than the opening 5 on account of the oblique deposition, the opening of the material layer 8 becomes smaller as the deposition proceeds. This makes the cathode 9 being formed on the substrate by deposition through the opening 5 become tapered off with time.)
Finally, the material layer 8 is removed by lift-off as the intermediate layer 7 is removed by etching with a sodium hydroxide solution which dissolves the intermediate layer 7 alone. Thus there is obtained a field emission cathode as shown in FIG. 11.
The thus formed field emission cathode emits electrons upon application of a voltage of about 10.sup.6 V/cm or above across the cathode 9 and the gate electrode (or the second electrode 3), with the cathode 9 unheated. This kind of minute field emission cathode can operate at a comparatively low voltage, with the gate voltage being of the order of tens to hundreds of volts. An array of hundreds of millions of such field emission cathodes arranged at intervals of about 10 .mu.m may be used as electron guns for a thin display that operates at a low voltage (or with a low electric power).
A disadvantage of the foregoing field emission cathodes is that the gate electrode 23 made of a high-melting metal such as molybdenum, tungsten, and chromium is liable to oxidation, which lowers its conductivity and hence leads to unstable electron emission.
Another disadvantage of the foregoing field emission cathodes is that the intermediate layer 7 made of aluminum or nickel is not completely removed from the gate electrode 23 by wet etching, but some residues (which are electrically conductive) remain undissolved. Residues remaining on the gate electrode 23 may adversely affect the electron emission characteristics and cut-off characteristics, or short-circuit the gate electrode 23 and the cathode 9. This leads to an increase in defective products and a decrease in yields.
The present inventors had previously proposed a process for producing an array of field emission cathodes without using the oblique deposition. (See Japanese Patent Laid-open No. 160740/1981.) This process consists of covering the obverse of a substrate of silicon single crystal with a masking layer having a patterned opening, performing crystallographic etching through the opening, thereby forming a conical hole, forming an electrode layer on the inside of the conical hole by vacuum deposition or sputtering of tungsten or the like, filling the conical hole with an insulating reinforcement material, performing ordinary etching (or non-crystallographic etching) on the reverse of the substrate (so that the apex of the electrode layer formed in the conical hole is exposed), thereby forming the tip of the cathode, forming an insulating layer so as to embed the cathode therein, and covering the insulating layer with a conducting layer. Finally, the conducting layer and insulating layer undergo etching as shown in FIGS. 13A and 13B, so that the cathode is exposed.
This process offers an advantage that the conical cathode invariably has an acute vertical angle and there are no problems involving the residues of the intermediate layer 7. However, there still remains the problem associated with the oxidation of the gate electrode which leads to a decrease in conductivity. The effect of oxidation is serious because the gate electrode is very thin (thousands of angstrom). The oxidized gate electrode will not operate satisfactorily with a gate voltage of the order of tens to hundreds of volts.
There is an alternative structure as shown in FIG. 15. It is characterized by a thin resistance layer 12 of silicon interposed between the first electrode 11 and the cathode 9. The resistance layer 12 has a thickness from several angstroms to several microns and also has a resistance of the order of hundreds to millions of .OMEGA..cm. The resistance layer 12 permits each cathode 9 to emit electrons at a constant rate. This will be described in more detail with reference to FIGS. 14 and 15 which are schematic enlarged sectional views showing an array of field emission cathodes.
Referring to FIG. 14, there are shown a plurality of cathodes 9.sub.1 and 9.sub.2 formed directly on the first electrode 11, which is not provided with the resistance layer 12. The electron flow is indicated by arrows e. In actual mass production of flat displays as mentioned above, the electrodes 9.sub.1 and 9.sub.2 will vary slightly in size and shape as shown in FIG. 14. This variation leads to the fluctuation of the electric field strength required for electron emission, which in turn causes the emissivity to fluctuate. For example, there would be an instance where the cathode 9.sub.1 emits electrons at 50 V, while the cathode 9.sub.2 needs 100 V for electron emission. There would be another instance where the cathode 9.sub.1 alone emits electrons at 50 V, while the cathode 9.sub.2 does not work at 50 V. There would be another instance where the cathode 9.sub.2 emits electrons at 100 V, while the cathode 9.sub.1 is broken at 100 V.
If a flat display is made up of field emission cathodes which are not uniform in shape as mentioned above, the screen will vary in brightness from one spot to another on account of the uneven electron emission. Moreover, the lack of uniformity causes some elements to be broken, which shortens the life of the flat display.
The foregoing problem does not arise from the field emission cathode as shown in FIG. 15. It has a resistance layer 12 interposed between the cathode and the first electrode 11. The resistance layer 12 gives rise to resistance R.sub.1 and R.sub.2 between the electrode 11 and the cathodes 9.sub.1 and 9.sub.2, respectively. It is assumed that when a voltage V.sub.0 is applied, the current i.sub.1 flowing to the cathode 9.sub.1 is larger than the current i.sub.2 flowing to the cathode 9.sub.2 so that the cathode 9.sub.1 emits more electrons than the cathode 9.sub.2. In this situation, the cathode 9.sub.1 experiences voltage drop due to the resistance R.sub.1, and hence the voltage applied to the cathode 9.sub.1 becomes EQU V.sub.1 =V.sub.0 -.DELTA.V.sub.1 =V.sub.0 -R.sub.1 i.sub.1
Similarly, the voltage applied to the cathode 9.sub.2 becomes EQU V.sub.2 =V.sub.0 -.DELTA.V.sub.2 =V.sub.0 -R.sub.2 i.sub.2
and V.sub.1 becomes smaller than V.sub.2. A moment later, the cathode 9.sub.1 emits less electrons than the cathode 9.sub.2. As the result, the emission of electrons from each cathode levels out. In this way, it is possible to keep uniform the screen of the flat display.
In addition, the resistance layer 12 prevents current from flowing freely from the tip of the cathode to the second electrode even when an electrically conductive minute particle of dust gets in between them, as shown in FIG. 16 which is a schematic enlarged sectional view. This situation permits adjacent cathodes to continue emitting electrons, with a prescribed voltage applied across the cathode and the second electrode.
However, the resistance layer 12 will not function properly if it has a defect such as a pinhole 20 as shown in FIG. 17, which is a schematic enlarged sectional view. In this case, the pinhole 20 connects the cathode 9 to the first electrode 11 and hence a short circuit takes place between the tip of the cathode 9 and the second electrode 3 when an electrically conductive minute particle of dust gets in between them. This situation prevents adjacent cathodes from emitting electrons.
The foregoing defect is liable to occur in a display composed of hundreds of millions of cathodes. In addition, short circuits by dust prevent a plurality of cathodes from emitting electrons and hence reduce the life of the display.